A microporous membrane filter is a thin sheet, generally formed from a synthetic plastic material, having a substantially uniform, continuous matrix structure containing millions of capillary pores. The pore diameters tend to be very uniform, within narrow limits.
A microporous membrane filter functions as an absolute screen or sieve. It retains on its surface all particles larger than the pore diameter. Many smaller particles will pass through the filter, but some particles, whose size approximately equals the pore diameter, may become trapped in the matrix. The net result of such entrapment may be that, if enough pores are blocked, the filter becomes plugged in an irreversible manner, and the flow rate declines.
Such microporous membrane filters are available with average pore sizes in the range from about 25 nanometers (0.025 micrometers or 250 Angstrom units) up to a preferred maximum size of about 14 micrometers. The term "microporous membrane" is not well defined in the art, however, and the upper limit pore size is considered by some to extend to about 20-25 micrometers. Under most circumstances, such filters will not retain even the largest of macromolecules. They are therefore not recommended for the technique known as "molecular filtration". However, the smallest of these pore sizes will retain viruses and significant fractions of many large macromolecules, and are therefore suitable for a limited number of certain molecular filtration applications.
Techniques for making microporous membrane filters can be found in each of the following U.S. Pat. Nos. 3,100,721; 3,208,875; 3,642,648; and 3,876,738. Thus, U.S. Pat. No. 3,100,721, describes a technique for making an unsupported microporous film from any one of a wide spectrum of polymers, including, among others, nylons, polyesters, vinyl polymers, and copolymers, and the like. The process involves coating a polymeric dispersion onto an expendible cellophane sheet, treating the coating with water, then drying the coating and stripping it from the cellophane sheet. This disclosure is incidental to the primary concern of the patent, which is the production of synthetic leather. The later, very similar U.S. Pat. No. 3,208,875, is also concerned primarily with the production of synthetic leather, and the production of a microporous film is incidental. The typical microporous stratum, for a synthetic leather product, has a porosity of from about 10% to about 20% by volume of the stratum.
U.S. Pat. No. 3,642,668 is concerned with the production of a particular kind of film for use in unique distillation equipment. The production technique described is the usual hand production technique where the polymer solution is coated on a glass plate, which is then immersed in a leaching bath. However, the patent speculates, beginning at column 5, line 67, that the polymer solution can be cast onto a rotating drum or belt, or even extruded directly into the leaching bath through a slotted extrusion die that is moved, relative to the bath, at a speed relative to the rate of extrusion. To be suitable for the intended purpose, the product is a microporous membrane having a pore volume of at least 50% by volume, and a majority of the pores have diameters in the range from 0.5 micrometers to 2.0 micrometers.
In U.S. Pat. No. 3,876,738, the polymer solution is cast directly under the surface of the leaching bath, so that it is not exposed to the atmosphere.
Attempts to produce microporous membrane filters having pore sizes in the true molecular size range, which is from about 10 Angstrom units up to about 100 Angstrom units, have generally led to problems of very slow flow and rapid plugging. Such pore sizes make molecular filtration possible, and generally have been achieved by the use of "skinned" membranes.
Skinned membranes differ in design and in performance from the microporous membrane filters discussed above. Such membranes have been known and understood at least since the publications of Loeb and Sourirajan, "Sea Water Demineralization By Means of a Semi-Permeable Membrane," U.C.L.A. Dept. of Engineering Report 60-60, 1960, and also, "Sea Water Demineralization By Means of An Osmotic Membrane," Advances in Chemistry Series, Vol. 38, 1962, pp. 117 et seq. Such membranes are now regarded as the filters best suited for retaining a wide range of macromolecules while maintaining high flow rates.
A skinned membrane consists of a thin polymeric film or skin that is supported on and integral with a highly porous substrate. The substrate contributes strength and durability to the filter, but the thin skin is the actual molecular filtration membrane, and it is placed on the upstream side, facing the fluid to be filtered. The skin layer is densely structured to be able to retain molecules, but is very thin, typically less than two micrometers. Becuase it is so thin, its resistance to flow is minimized. Since the skin is backed by a very open, porous substrate layer, flow rates through the membrane are high. Retained molecules and particles are held at the surface of the membrane, on its skin, and do not enter into the porous structure. Skinned membranes therefore seldom become plugged.
A microporous membrane filter is generally given an absolute pore size rating, and it will retain all particles larger than that pore diameter. However, a skinned membrane, at least in its skin portion, does not have discrete pores that can be measured accurately. Skinned molecular filters retain most molecules above a nominal or approximate limit, as well as some fraction of smaller molecules. They do not retain all molecules larger than an absolute cutoff size. Since some dissolved macromolecules apparently deform or are forced through the skin, and because of the complicated character of the skin, only nominal limits are appropriate, to characterize membranes of this kind. Since molecular weight is an approximate guide to molecular size, it is convenient to characterize molecular filters by their percent retention of selected solutes having accurately known molecular weights. From such data, a membrane can be assigned a nominal molecular weight limit. It represents the molecular weight at and above which most species are retained by that membrane. This limit is most reliable as a guide with respect to relatively globular molecules.
A typical process for the production of a skinned molecular filter, sometimes called a reverse osmosis membrane, is described in U.S. Pat. No. 3,412,184. That process involves casting a solution of a cellulose ester as a thin film, evaporating a portion of the casting solvent, leaching with an organic solvent, optionally immersing in a hot water bath to subject the film to a heating step, and then recovering the product membrane. The production technique is essentially a manual technique. In Example 1, it is suggested that the cast film be leached in methanol, and then transferred successively into solutions of methanol and water having weight ratios of 80:20, 60:40, and 20:80 parts, respectively, and then into a bath of essentially pure water. The heating step, in the water bath, was for the purpose of developing desalination characteristics.
A more advanced, continuous process for the production of cellulose ester membranes is described in U.S. Pat. No. 3,792,135. This process involves the following steps, in the order stated:
(1) coating a film of a "dope" of a cellulose ester on a web;
(2) permitting solvent to evaporate into the atmosphere, to cause incipient formation of a skin or "active" layer; and
(3) immersing the film in a hot aqueous bath, to gel the film in the form of an asymmetric membrane.
A more recent U.S. Pat. No. 3,988,245, describes a skinned membrane made of a polyvinyl formal resin and cast on a cloth backing, and a solvent-non-solvent process for making it. This patent illustrates the typical scanning electron microscope cross-section, at 10,000 times enlargement, for such a membrane. That cross-section is characterized by somewhat tubular pore walls that extend transversely of the membrane, that is, in a direction generally perpendicular to the skin. In FIG. 1 of the patent, these tubular, somewhat cylindrical walls extend between the skin, on one face of the membrane, and the cloth backing, on the other face of the membrane.